The gas permeability of polymer films is very important in a variety of packaging applications. For example, for many food and beverages, the package's resistance to oxygen and water intrusion is the limiting factor for their shelf lives. Today, new and more stringent packaging standards are being issued to address the public concerns on the health safety, energy, and environmental issues related to plastics packaging. This has triggered a new wave of development of novel gas barrier films, usually with aims for lower gas permeability. At the same time, the demand for fast, accurate, and versatile gas permeation test systems is also on the rise. However, it becomes increasingly difficult to quickly and accurately characterize the permeabilities of new film products as the permeabilities are being reduced to unprecedented low levels. Hence, there is also an acute need to advance the state of the art of gas permeability measurement in parallel with the material development.
Experimental methods for permeation rate measurement have undergone development for more than a century. The two general methods are the variable-pressure (manometric) method and the variable-volume (volumetric) method. Both have been standardized by American Society of Testing Methods (ASTM) since the 1950s such as in ASTM method D1434-82, which is incorporated by reference herein. In the manometric method, a gas permeates through a film into a closed constant-volume chamber that is pre-evacuated. The pressure rise in the chamber is recorded as a function of time by reading displacement of mercury in a capillary (manometer). In the volumetric method, the chamber into which a gas permeates is allowed to expand against a low constant pressure (usually atmospheric). The volume change of the chamber is recorded as a function of time by reading displacement of a liquid in a capillary. The two methods provide for the determination of steady-state gas permeation rate, permeance (i.e., the ratio of the gas permeation rate to the difference in partial pressure on the two sides of the film), and, in the case of homogenous materials, permeability. The repeatability and reproducibility of the two methods are satisfactory and the data agreement between the two methods is also good. For these reasons, the two methods have been widely used.
However, the manometric and the volumetric methods have two disadvantages. First, these methods record integrated information (pressure or volume), rather than differential rate information. An experimental curve recorded in this way has transient and steady state components. The transient part precedes the steady state and is shown as a nonlinear pressure rise with time. At the steady state, the change of pressure with time becomes linear. There is a time lag between the time when the penetrant enters the test film and the time when the permeation process reaches the steady state. The diffusion coefficient, solubility coefficient and permeability coefficient of the test film can be determined by correlating the observed time-lag with mathematical diffusion models. This technique is called the time-lag analysis, which is the only viable analysis for the determination of the transport coefficients for the two methods. Though in theory transient permeation rates can be obtained by differentiating a time-lag curve with respect to time, it is not recommended for practice because numerical differentiation is prone to error and the worst signal-to-noise ratio is always found in the primary region of interest (i.e., where the nonlinear rise developed just as the curve departs from the baseline prior to its linear growth). Consequently, the manometric and the volumetric methods give apparent properties rather than intrinsic ones. For example, if the diffusion coefficient of the test film is non-constant in the process, the use of the time-lag technique will lead to an apparent diffusion coefficient which may be significantly different from the intrinsic diffusion coefficient. Moreover, the permeation test has to be carried out to the steady state. The time required to reach steady state will depend on the nature of the specimen, its thickness, and the applied pressure differential. For specimens of low permeability, long periods of test and repeated measurement may be required to obtain reliable results. Second, as the methods of recording pressure change or volume change are indifferent to gas composition, individual gas permeation in a gas mixture cannot be differentiated by either of the methods. Hence, the manometric and the volumetric methods are ideal for studying pure gas permeation only. There can be a question as to whether the permeability determined for pure gases can be used for multi-component gas permeation processes. Therefore the application range of the manometric and volumetric methods is rather limited.
Clearly, there is a need to measure transient permeation rate so that intrinsic properties can be obtained and test period can be shortened. There is also a need to overcome the lack of selectivity in the manometric and the volumetric methods. The problems can be solved by choosing a suitable gas detecting method in place of the old pressure- or volume-recording method. In literature, usages of thermal conductivity detector, coulometric detector, infrared spectrometer, gas chromatograph (GC) and mass spectrometer (MS) have been reported for measurement of gas permeability of polymers. For example, various measurement methods are described based on the principle of the thermal conductivity of gas mixtures. Though gas transmission rate may be automatically recorded by these methods, the usage of thermal conductivity detector alone does not solve the selectivity problem. Coulometric detectors are used in commercial devices sold by Mocon Inc. for measuring oxygen and water vapor transmission rates, respectively. Infrared water vapor sensors are also used by the same company for measuring water vapor transmission rates. Again, these commercialized methods lack the selectivity in gas measurement as one type of sensor can only detect one specific type of gas. In terms of selectivity, however, GC and MS are probably the most promising techniques.
GC has been used for the analysis of multi-component gas permeation. In common the current methods use an ionic pressure gauge or a thermal conductivity detector to continuously monitor the change of the total gas pressure and use GC to analyze gas composition. The major difference lies in how the permeation cell is designed to interface with GC. There are two major types of permeation cells: continuous flow cell and vacuum cell. The continuous flow cell includes two compartments separated by a test film. The test gas is introduced into one compartment and interacts with the upstream face of the test film; a carrier (or sweep) gas flows at constant rate in the other compartment and interacts with the downstream face of the test film. The permeant that diffuses through the test film is swept by the carrier gas and transferred to the gas detector relatively far downstream. Compared with the vacuum cell, the continuous flow cell has some advantages: (1) little or no film support is required as pressures can be balanced between the two compartments; (2) leakage should have a minimum effect on the testing results; (3) the conditioning time may be shortened as there is no need for degassing. As carrier gases are used in the GC technique, it is relatively easy to place a GC at the downstream of a continuous flow cell and use the same carrier gas for both the permeation cell and the GC. However, accurate control of the carrier gas flow rate is important, and undesirable back diffusion of carrier gas can occur to a measurable extent. To avoid using a downstream sweep gas, some methods use a vacuum cell design, with the downstream compartment pre-evacuated. Since such a vacuum cell is a variant of the manometric vacuum cell, it has the same disadvantages, such as film distention/rupture, leak, or the like. The common problem with these GC methods is that the gas composition analysis is operated in a batch mode rather than a continuous mode as the rate of composition analysis is restricted by the response time of a GC and the length of time required to complete a gas chromatographic analysis of a potentially complex gas mixture (typically 5-20 minutes). Therefore, it is doubtful that the GC methods can effectively measure transient permeation rates particularly when the rates are high. Moreover, the GC streams must be accurately controlled and carefully calibrated for each separation scheme and each operating condition.
Mass spectrometers used for gas analysis are commonly referred to as residual gas analyzers. It is desirable to have a single source detector that can measure partial pressures quickly. A residual gas analyzer (referred to herein interchangeably as “residual gas analyzer”, “mass spectrometer”, or “MS”) offers this advantage for a wide range of gases. It allows the partial pressures of gas components to be determined simultaneously, an operation which is not possible with either a GC or an absolute pressure gauge. Because a MS must operate at high vacuum conditions, traditionally the modified vacuum cell was used to couple with MS. Designs have been described for pervaporation. In such designs, an absolute pressure gauge was used to monitor the vapor pressure in the pre-evacuated downstream compartment. A small aperture of known area was placed between the compartment and an MS, which restricts the molecular flow rate and reduces the leak rate to a negligible level compared to the pressure before the aperture. The permeation transient was calculated from an empirical formula once the pressure before the aperture, the aperture area and gas property are known. However, suppose the pressure after the aperture is 10−6 torr, the pressure before the aperture must be 10−3 torr or greater in an ante chamber to apply the formula. If the volume of this ante chamber is too large, the residence time of permeants in this ante chamber at 10−3 torr can be so long that transient rates are not attainable; this is known as a memory effect, and refers to the length of time between permeation and detection. This implies that the estimation of the permeation transient in the initial stage is not likely accurate. Nevertheless, it has been concluded that the total pressures as measured by the MS and by the pressure gauge agree well enough and hence the measurement of permeation transient should be possible. Similar designs have been used for the pervaporation process. Note that with such designs, the pressure before the aperture is accumulated during the measurement, meanwhile the pressure after the aperture changes accordingly. Therefore, error will be introduced into the calculation of the molecular flow rate at the end of the process. Moreover, a potential risk with the vacuum cell design is that the MS may be over-pressured, either because of the accumulation of pressure or because of an accidental film rupture event. Protection must be taken, usually with the aid of a bypass valve set at a safe pressure. Consequently, it is not always possible for a permeation experiment to reach steady state. Recently, designs have been described in which the absolute pressure measurement and the MS detection occurred in the same high vacuum chamber. Such designs have a higher risk of over-pressuring the MS. As a result, impractically low pressures (10−5 to 0.13 atm) have been applied on the upstream side of the specimen film in such designs.
In contrast to a vacuum cell, the pressure in a continuous flow cell can be maintained at a steady value, as can the pressure before the MS. This avoids the complications in a vacuum cell as discussed above. The major concern with the continuous flow cell is the dilution effect by the sweep gas, which may lower the permeant concentration to an undetectable level. So far, coupling of MS and the continuous flow cell has rarely been seen.
Thus, a need exists for improved methods for measuring permeation rates. In particular, any method that can measure permeation rates of either pure or mixture gases would also be desirable. Systems for carrying out such methods would be particularly beneficial.